Negative‐ion extraction of gaseous materials from a radio frequency plasma‐sputter‐type heavy negative‐ion source

The operational characteristics of a rf plasma‐sputter‐type heavy negative‐ion source with a feeding of O2 or SF6 gas as an ionized material together with Xe gas is presented. To obtain negative ions of chemically reactive elements, we used a material gas and a stainless‐steel sputtering target instead of an oxide or fluoride sputtering target to prevent charging trouble. In the source, gas particles adsorbed on the target surface were negatively ionized by sputtering. O− or F− was dominant in the extracted beam and increased with the Cs supply until the yield was optimized. Then, high‐current negative ions of mA at an intensity of several, such as 4.6 mA for O− and 4.3 mA for F−, were extracted in a dc mode of operation. Even in this plasma‐sputter source with a material gas feeding, the surface production of negative ions was found to be the dominant mechanism.